Printed circuit assembly (PCA) test hardware is used to test PCAs after manufacture. Such testing may include electrical connectivity, voltage, resistance, capacitance, inductance, circuit function, device function, polarity, vector, vectorless, and circuit functional testing.
In order to perform the tests, the tester hardware must be capable of probing conductive pads, vias and traces on the board under test. Electronic signals are employed by the board test system to determine whether each electronic component on the PCA is operating properly. A test fixture provides a mechanical and an electrical interface between the board test system interface pins and the electronic components located on a PCA to be tested. Since signals must pass through the test fixture both on their way to and from the electronic component, the test fixture must not degrade the quality of these signals in order to ensure that the electronic component is correctly diagnosed as operating properly or improperly.
In order to ensure a high quality signal path to and from the test system, the probes must make a firm electrical and mechanical contact with the component. This is commonly achieved using vacuum-actuated test fixtures.
Prior art vacuum actuated test fixtures are typically constructed from two plates. The first plate, or probe plate, is a thick plate made from an insulator having holes corresponding to the locations of the electrical nodes/contacts of interest on the PCA under test. The probes are mounted in these holes. Typically the probes comprise a spring loaded probe and a probe socket. A second plate, or top plate, having holes corresponding to the locations of the probes, is mounted on alignment pins and held above the probe plate by preloaded fixture springs. A seal is then placed around the perimeter of the plates to form a vacuum chamber. When air is removed from the vacuum chamber, the top plate is drawn toward the probe plate causing each probes to pass through its corresponding hole in the top plate and strike electrical nodes/contacts of interest on the PCA under test. The PCA under test is positioned relative to the top plate by two or more tooling pins and held in place by the vacuum.
Single plate fixtures have also been developed. These fixtures comprise a probe plate which is almost identical to the probe plate of the two plate fixture. A thick layer of foam is placed directly on top of the probe plate. The foam is removed from the probe plate directly under the PCA except for the approximately one-half inch around the perimeter of the PCA which serves as the vacuum seal. Accordingly, the PCA itself operates as the top plate of the vacuum chamber.
In each embodiment of vacuum-actuated test fixtures, the fixture probes (i.e., the probes that make electrical contact with nodes on the PCA under test) lie within the vacuum chamber during test. Although the electrical signals theoretically pass from the fixture probe to its corresponding node on the PCA, and/or vice versa, such that one could measure the signal on the probe through the tester, occasionally it is desirable and useful to make measurements directly on the probe itself. For example, during debug of the PCA, one may desire to ensure that a signal generated by the tester is actually being delivered by the probe to the desired point of contact on the PCA. If not, this could indicate a fault in the tester. If so, it could indicate a fault on the PCA or insufficient contact force between the probe and probed PCA node. Conversely, it may be useful to determine whether a signal expected to be generated by the PCA is being received by a given probe. If not, it could indicate insufficient contact force between the probe and probed PCA node, or a fault on the PCA. If so, it could indicate a fault in the tester. Many other scenarios exist where it would be useful to allow an external instrument such as an oscilloscope, multimeter, or logic analyzer to connect to a probe within the vacuum-sealed chamber of the test fixture.
However, because the fixture probes and/or other electrical nodes of interest are sealed within the vacuum chamber of the fixture, connection to such probes/nodes by external electronic instruments is problematic due to probe access difficulties.
One method of probe access is to machine holes in the vacuum chamber enclosure (e.g., probe plate, PCA, vacuum chamber cover or vacuum seal). However, manufacturing vacuum enclosures with holes or apertures generally complicates the design and implementation of the vacuum fixturing and/or PCA due to the complexity in determining suitable locations for external instrument probe holes and the difficulty associated with maintaining a proper seal. Accordingly, this method is complicated, expensive, and time consuming.
Another method for achieving probe access by external instrument probes is to connect the external instrument probe to the desired fixture probe prior to actuation of the vacuum. The cable of the external instrument probe is then manually routed between the vacuum seal and appropriate fixture component (e.g., probe plate). Then, when the vacuum is actuated, the vacuum seal presses against the fixture with the external instrument probe cable wedged therebetween. This technique is advantageous over the above-described holing method in that the complexity and expense of designing and machining the holes is eliminated. However, the cable routing method is problematic in that the probe cable interferes with and prevents the creation of a true vacuum seal. FIG. 1, which shows a cross-sectional view of a test instrument cable 200 wedged between a vacuum seal 202 and fixture 204, illustrates the problem. As illustrated, the vacuum seal 33 presses against the probe plate of the fixture 21 over the external instrument cable, creating air gaps 215 on either side of the cable 200. The cross-sectional diameter dc of the probe cable, which is typically on the order of 0.125 inch, is large enough that the air gaps 205 allow sufficient leakage to prevent the creation of a true vacuum seal. This also risks damage to the probe cable, which is often quite expensive.
The current methods for probing a vacuum-sealed fixture probe and/or other nodes of interest sealed within the vacuum chamber are either very costly, or sacrifice vacuum-induced pressure on fixture-probe-to-PCA-node contacts due to air leakage through gaps created by external instrument cable routing. Accordingly, a need exists for an improved technique for allowing connection of test probes of external electronic instruments to electrical probes and/or other nodes of interest sealed within a vacuum chamber.